Optically encoded information, such as barcodes, have become common. A barcode symbol consists of a series of light and dark regions, typically in the form of rectangles. The widths of the dark regions, the bars, and/or the widths of the light spaces between the bars indicate the encoded information. A specified number, widths, and arrangement of these elements represent a character. Standardized encoding schemes specify the arrangements for each character, the acceptable widths and spacings of the elements, the number of characters a symbol may contain or whether symbol length is variable, etc. The known symbologies include, for example, UPC/EAN, Code 128, Codabar, and Interleaved 2 of 5.
Readers and scanning systems use electro-optical means to decode each symbol, thus providing multiple alphanumerical characters that typically are descriptive of the article to which the symbol is attached or some characteristic thereof. Such characters are typically represented in digital form as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,360,798; 4,369,361; 4,387,297; 4,409,470, and 4,460,120, all of which have been assigned to Symbol Technologies, Inc.
To decode a barcode symbol and extract a legitimate message using such optical scanners, a barcode reader scans the symbol to produce an analog electrical signal representative of the scanned symbol. A variety of scanning devices are known. The scanner could be a wand type reader including an emitter and a detector fixedly mounted in the wand, in which case the user manually moves the wand across the symbol. Alternatively, an optical scanner scans a light beam such as a laser beam across the symbol, and a detector senses the light reflected from the symbol.
In either case, the detector senses reflected light from a spot scanned across the symbol and provides the analog scan signal representing the encoded information.
A digitizer processes the analog signal to produce a square wave where the widths and spacing between the square boxes correspond to the width of the bars and the spacing between bars. The digitizer serves as an edge detector or wave shaper circuit, and the threshold value set by the digitizer determines what points of the analog signal represent bar edges. The threshold level effectively defines what portions of a signal the reader will recognize as a bar or a space.
The signal from the digitizer is applied to a decoder. The decoder first determines the square shape widths and spacings of the signal from the digitizer. The decoder then analyzes the widths and spacing to find and decode a legitimate barcode message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard the scanned symbol conforms to. This recognition of the standard is typically referred to as autodiscrimination.
Different barcodes have different information densities and contain a different number of elements in a given area representing different amounts of encoded data. The denser the code, the smaller the elements and spacings. Printing of the denser symbols on an appropriate medium is exacting and thus is more expensive than printing low resolution symbols. The density of a barcode symbol can be expressed in terms of the minimum bar/space width called also "module size" or as a "spatial frequency bandwidth" of the code, which is the inverse of twice the minimum bar or space width.
A barcode reader typically will have a specified resolution often expressed by the module size that is detectable by its effective scanning spot. For optical scanners, for example, the beam spot size is larger than approximately the minimum width between regions of different light reflectivities, i.e., the bars and the spaces of the symbol. The resolution of the reader is established by parameters of the emitter or the detector, by lenses or apertures associated with either the emitter or the detector by angle of beam inclination, by the threshold level of the digitizer, by programming in the decoder, or by a combination of two or more of these elements. In a laser beam scanner the effective sensing spot typically corresponds closely to the size of the beam at the point it impinges on the barcode. The photodetector will effectively average the light detected over the area of the sensing spot.
The distance within which the barcode scanner is able to decode a barcode is called the effective working range of the scanner. Within this range, the spot size is such as to produce accurate readings of barcodes for a given barcode line density. The working range relates directly to the characteristics of the scanner components and to the module size of the barcode.
Typically, an optical barcode scanner includes a light source, such as a gas laser or semi-conductor laser, that generates the light beam. The light beam is optically modified, typically by a lens, to form a beam spot of a certain size at a prescribed distance. The optical scanner further includes a scanning component and a photodetector. The scanning component sweeps the beam spot across the symbol. The photodetector has a field of view which extends across and slightly past the symbol and functions to detect light reflected from the symbol. The electrical signal from the photodetector is converted into a pulse width modulated digital signal, then into a binary representation of the data encoded in the symbol, and then to the alphanumeric characters so represented, as discussed above.
Many known barcode scanning systems collimate or focus the laser beam using a lens system to create a beam spot of a given diameter at a prescribed distance. The intensity of the laser beam at this point, in a plane normal to the beam (i.e., parallel to the symbol), is ordinarily characterized by a "Gaussian" distribution with a high central peak. The working range is defined as the region within which the intensely bright beam spot can decode the information after being scanned across the barcode symbol. One desires a large longitudinal distance within which range the designed beam allows barcode patterns to be accurately scanned. But as the distance between the scanner and the symbol moves out of the working range of the scanner, which is typically only a few inches in length for most common barcode densities, the Gaussian distribution of the beam spot greatly widens as a result of beam diffraction, preventing accurate reading of a barcode. This widening effect is more pronounced for narrow "pencil" beams, necessary for scanning fine barcode patterns. The laws of physical optics predict that a uniform beam with a circular aperture of radius "a" will spread in free space within a cone with a half angle of 0.61.lambda./a, where is wavelength of the beam.
The far field region, where the beam spreads at this rate, starts at the distance EQU z=.pi.a.sup.2 /.lambda.
For a beam of Gaussian profile, the field amplitude distribution in the plane of its waist (narrowest region) is exp(.sup.-r .sup.2 /.omega..sup.2.sub.o), where .omega..sub.o is the waist radius. Such a beam spreads with a half angle of .lambda./.pi..omega..sub.o before and after a corresponding Rayleigh distance (or confocal beam parameter) of H=.pi..omega..sup.2.sub.o /.lambda..
Because the requirements of the scan beam to be of narrow diameter and to be maintained at uniform diameter for a long distance are contradictory, present scanning systems must be positioned within a relatively narrow range of distances from a symbol in order to properly read it. For example, a scanning beam with a wavelength .lambda. of 0.67 .mu.m, and an aperture of 15 mils or 0.38 mm (=2.omega.o) will provide a working range (=2H) of 340 mm. Finer beams of smaller diameter will have much shorter working ranges.
It has been recently shown (J. Durnin, Exact Solutions for Nondiffracting Beams, JOSA A, 4, 651 (1987), also U.S. Pat. No. 4,852,973), that a beam with an amplitude profile given by the Bessel function of zero order J.sub.o (.alpha.r), r denoting the transverse distance propagates without expansion. It is obvious that such a beam is unrealizable in a practical optical system, due to its infinite lateral extent. The energy is spread out, so that same energy is contained in rings of equal width. Such a beam cannot be used for barcode scanning due to the infinitely wide spread of energy in its cross-section, as a result of which the detected signal has very low contrast.
Nevertheless, it has been shown that a J.sub.o (.alpha.r) distribution can be generated by a circular fan of a multitude of plane waves propagating at an angle .theta. with respect to the z-axis, i.e. by integrating all plane waves propagating at an angle .theta. with respect to the z-axis. One device that comes "close" to that distribution is the "axicon," which indeed generates a circular fan of semi-plane waves, but those are not of infinite extent.
It has been shown that axicons provide a limited region where the "quasi J.sub.o " distribution is obtained (see J.H. McLeod, "Axicon: A new type of optical element", JOSA A, 44, 592 (1954)), G. Indebetouw, "Nondiffracting optical fields . . . ", JOSA A, 6, 150 (1989)), A. Vasara et al., "Realization of general nondiffracting beams . . . ", JOSA A, 6, 1748 (1989)). Scanning optics implementing the axicon are described in Katz et al., U.S. Pat. No. 5,080,456 and Marom et al. copending application Ser. No. 07/936,472, filed Aug. 28, 1992, both assigned to Symbol Technologies, Inc.
An axicon has a region given by a/.theta.=(n-1).alpha. whereby a "quasi-Bessel distribution" is generated. In this relation, a=aperture radius, n=index of refraction of the axicon material, .alpha.=angle between surfaces of the axicon and .theta.=resulting phase front tilt. Although the extent of the beam is limited in its cross-section, the quasi-Bessel distribution achieved is similar to the ideal "non diffractive" beam, thus also suffering from same poor signal contrast, as mentioned above.